The invention pertains to electrical temperature sensing devices of the type that exhibit a varying resistance characteristic as a function of temperature and, in particular, that are employed in food heating appliances for monitoring the heating operation. The variations in electrical resistance may be translated by appropriate circuit means into electrical control and/or indicator signals. It is desirable that such devices possess good thermal response characteristics so as to closely track the heating and cooling sequences that occur in food heating appliances and provide accurate resistance variations in response to temperature change. Another desirable property is that they have mechanical strength appropriate for mounting and use within food heating compartments.
One common temperature sensing device is the bead thermistor. This device consists of a bead of semiconductor material exhibiting a negative temperature coefficient of resistance, the bead being fixed between two straight wire leads for reducing the resistance between these wire leads as the temperature increases. Due to the mass of its body, the device does not have thermal response characteristics adequate for responding to rapid heating and cooling occurrences. In addition, a device of this type is not suitable for mounting within a food heating appliance.
There are a number of different constructions of temperature sensing devices having the sensor element in the form of a temperature sensitive resistance wire wound about an insulating core member, and having an outer enclosure. It is common to electrically insulate the wire element by coating the wire or embedding it in an insulating medium such as a refractory powder or a ceramic cement. However, for devices that are dimensioned for good thermal response this form of construction provides inadequate insulation. It is found that none of the existing devices of this kind optimumly combine the above noted properties of thermal response, temperature sensitive resistance and mechanical strength for application to food heating appliances such as toasters and the like.